Prévost Reaction

The Prévost
Reaction allows the synthesis of anti-diols from alkenes by the
addition of iodine followed by nucleophilic displacement with benzoate
in the absence of water. Hydrolysis of the intermediate diester gives
the desired diol.

Mechanism of the Prevost Reaction

The initial addition of iodine leads to a cyclic iodonium ion, which is
opened through nucleophilic substitution by benzoate anion:

A neighbouring-group participation mechanism prevents the immediate
nucleophilic substitution of iodine by a second equivalent of benzoate that
would lead to a syn-substituted product. Instead, a cyclic benzoxonium
ion intermediate is formed:

Opening of this intermediate by a second addition of benzoate gives the
anti-substituted dibenzoate:

Hydrolysis then delivers the diol.

In the Woodward-Modification,
added water decomposes the above benzoxonium intermediate directly to a syn-substituted
diol.

The use of expensive silver salts, the requirement for a stoichiometric
amount of molecular halogen, and the formation of a relatively large amount of
organic and inorganic wastes are definite drawbacks to this reaction. Sudalai
recently reported on catalytic versions of both the Prévost Reaction and the
Woodward-Modification.

The proper choice of stoichiometric oxidant allows either syn- or
anti-selective dihydroxylations. NaIO4 as the oxidizing agent
generates H2O as a side product of the oxidation and therefore
enables the Woodward Reaction to take place.

High-valent iodine reagents are still relatively expensive, and the
identification of a less costly stoichiometric oxidant would be a significant
improvement for this process.